The semiconductor material which is at present mostly used as the material for discrete semiconductor devices, e.g. transistors or diodes and for integrated circuits, e.g. logic devices or memory devices is silicon (Si). Besides, compound semiconductors of the groups III-V on the periodic table, e.g. gallium arsenide (GaAs) or indium phospher (InP) have been applied to special, restricted region of semiconductor devices, e.g. optoelectronic devices or super high frequency integrated circuits, because of the high electron mobility or the direct transition between the conduction band and the valence band.
Although silicon and gallium arsenide are excellent materials for semiconductor devices, these materials are not immune from the common drawback that the semiconductor devices made from these materials cannot be used at high temperature. The silicon devices cannot work above 200.degree. C. Even the gallium arsenide devices cannot work above 300.degree. C. This drawback is caused by the narrow band gaps between the conduction band and the valence band, e.g. 1.1 eV for silicon and 1.5 eV for gallium arsenide (1 eV=1.602.times.10.sup.-19 Joule). Above the temperatures (200.degree. C. for silicon and 300.degree. C. for gallium arsenide), the silicon or gallium arsenide semiconductor devices enter into the intrinsic region with big carrier densities.
The "carrier" means either or both electrons and holes. The "carrier density" is defined by the number of carriers in unit volume. The word "intrinsic" has a special meaning here. It is well known that semiconductors are classified into three kinds of semiconductors with regard to the electronic property; a p-type, an n-type and an intrinsic semiconductors. The p-type semiconductor has holes as majority carriers and electrons as minority carriers. The Fermi level is lower than the middle point between the bottom of the conduction band and the top of the valence band. For example, the silicon doped with boron (B) is a p-type semiconductor. The n-type semiconductor has electrons as majority carriers and holes as minority carriers. The Fermi level is higher than the middle point between the bottom of the conduction band and the top of the valence band. For example, the silicon doped with phosphor (P), arsenide (As) or antimony (Sb) is an n-type semiconductor. The intrinsic semiconductor has nearly equal number of holes and electrons, which are not called majority carriers nor minority carriers. The Fermi level coincides with the middle point between the bottom of the conduction band and the top of the valence band. In spite of the difference of electronic property, the product of the electron density and the hole density is a constant value which depends solely on temperature. Then, a person skilled in silicon semiconductor devices surely considers the intrinsic semiconductor has high resistivity because of the low carrier densities, that is, low electron density and low hole density. However, the meaning of the sentence that the silicon semiconductor enters the intrinsic region is totally different from the common sense of the person skilled. Here, the intrinsic region is used as a semiconductor which has nearly same densities of electrons and holes, but the carrier densities are very high, because the products of the densities increase according to rising of temperature. In spite of the carriers which have been supplied by an n-type dopant or a p-type dopant, thermal agitation overwhelmingly supplies many electrons and holes by exciting electrons from the valence band to the conduction band. Thus, the electron density and the hole density become almost equal in both the originally n-type semiconductor and the originally p-type semiconductor. Then, it is expressed by "entering the intrinsic region or intrinsic state". The thermal agitation easily lifts up the electrons over the band gap in silicon or gallium arsenide semiconductor devices, because of the narrow band gaps (1.1 eV for Si and 1.5 eV for GaAs). If the semiconductor devices are heated above the critical temperatures (200.degree. C. for Si and 300.degree. C. for GaAs), they cannot work, because a pn junction of a diode or a bipolar transistor cannot check reverse current flowing from the n-type region to the p-type region, an electric field applied at a gate of a FET (field effect transistor) cannot make depletion layer where no carrier exists. All layers of the semiconductor devices become the regions of low resistivity. Big currents flowing in the devices generate great amount of heat, which breaks down the devices in a short time.
In addition, since the degree of integration of silicon integrated circuits has been increasing year by year, the heat generation per unit volume of semiconductor devices is increasing also. The great heat generation coupled with poor heat diffusion would heat the device above the critical temperature, which may cause the disorder or breakdown of the devices. One method for resolving the heat generation in the highly integrated circuit is through the facilitation of heat diffusion or radiation using heat sinks with high heat conductivity, cooling fan or water cooling.
However, another solution of the heat problems has been proposed. The solution is fabricating semiconductor devices themselves with diamond. For example, Japanese Patent Laying Open No. 59-213126 (213126/'84) and Japanese Patent Laying Open No. 59-208821 (208821/'84) have proposed diamond semiconductor devices which would excel in heat diffusion and heat resistance. Diamond has many advantages as a semiconductor material. First, diamond is chemically very stable. Second, because of the wide band gap (5.5 eV), the temperature region in which a p-doped or an n-doped diamond is converted into an intrinsic semiconductor does not exist below 1400.degree. C. Below this temperature, the diamond is thermally stable, since it is neither melted nor evaporated in nonoxygen atmosphere. Third, the diamond enjoys high heat diffusion, because the heat conductivity of diamond is 20 W/cm K, which is more than ten times as large as that of silicon. Fourth, diamond is gifted with high carrier mobilities. At 300K (Kelvin: absolute temperature), the electron mobility is 2000 cm.sup.2 /V sec, and the hole mobility is 2100 cm.sup.2 /V sec. High carrier mobility would bring about high frequency analog devices or high speed logic devices. Fifth, diamond has a large dielectric constant K=5.5. Sixth, diamond is endowed with a large breakdown electric field E.sub.B =5.times.10.sup.6 V/cm. Therefore, it is expected that the semiconductor devices which excel in heat resistivity, work at high temperature under severe environment or generate output signals with high electric power will be fabricated by using diamond as a material of the semiconductor devices.
Preferably, the diamond as a material of semiconductor devices should be a single crystal. Today, the chemical vapor deposition method (CVD) enables us to grow a diamond single crystal epitaxially on a diamond substrate or on a silicon substrate by exciting the mixture gas consisting of methane (CH.sub.4) and hydrogen (H.sub.2) into plasma by microwave oscillation etc. Furthermore, an n-type diamond or a p-type diamond can arbitrarily be produced by doping pertinent dopants, such as B (p-type dopant) or P (n-type dopant) during the epitaxial growth. A non-doped diamond is an insulator with high resistivity.
However, the electric property of the diamond semiconductor layers fabricated by the CVD method heavily depends on the order of crystal. The order of crystal means the degree of the regularity of lattice in a crystal. Poor order of crystal means the state of crystal having high density of lattice defects. The poor order of crystal lowers the carrier mobilities, because the lattice defects scatter the carriers many times.
Especially, the diamond layer doped with some dopants suffers from great amount of lattice defects more heavily than the non-doped diamond layer. Therefore, when a pn junction of diamond layers or a Schottky junction of metal and diamond layers is fabricated to make a diode, a bipolar transistor or field effect transistors, many parasitic surface or interface states occur owing to the highly populated lattice defects. Here, the surface or interface state means an electronic or a hole state at the surface or the interface of the junctions where a lattice defect captures an electron or a hole at a certain energy in the band gap. In an ideal crystal without defects, the band gap defined as the region between the top of the valence band and the bottom of the conduction band has no electronic or hole state. Then the band gap is often called a forbidden band.
The surface or interface states induced by the lattice defect bring about new energy levels that electrons or holes can occupy in the band gap. The "surface or interface" means that the level is generated in the vicinity of the interface between the n-type layer and the p-type layer or between the semiconductor layer and the metal layer. It does not mean that the energy level is near the conduction band or near the valence band.
The higher the dopant concentration becomes, the larger the reverse leakage current flows from the n-type layer to the p-type layer through the intermediary of the surface or interface levels. Thus, heavy doping deteriorates the rectifying property of diode by the occurrence of surface or interface levels. Of course, the break down voltage of diode is lowered, because the leakage current generates big amount of heat which would often break the pn junction or the Schottky junction.
A purpose of the invention is to provide a diamond semiconductor device containing a pn junction or a Schottky junction with low reverse leakage current, high break down voltage and excellent rectifying property. Here, either the entire device or just the active parts of the device are made from semiconductor diamond.